Exhaust hoods are used to remove air contaminants close to the source of generation located in a conditioned space. For example, one type of exhaust hoods, kitchen range hoods, creates suction zones directly above ranges, fryers, or other sources of air contamination. Exhaust hoods tend to waste energy because they must draw some air out of a conditioned space in order to insure that all the contaminants are removed. As a result, a perennial problem with exhaust hoods is minimizing the amount of conditioned air required to achieve total capture and containment of the contaminant stream.
Referring to FIG. 1A, a typical prior art exhaust hood 45 is located over a range 40 or other cooking source. The exhaust hood 45 has a recess 25 with at least one vent 20 (covered by a filter also indicated at 20) and an exhaust plenum 20 and duct 10 leading to an exhaust system (not shown) that draws off fumes 35. The exhaust system usually consists of external ductwork and one or more fans that pull air and contaminants out of a building and discharge them to a treatment facility or into the atmosphere. The recess 25 of the exhaust hood 45 plays an important role in capturing the contaminant because heat, as well as particulate and vapor contamination, are usually produced by the contaminant-producing processes. The heat causes its own thermal convection-driven flow or plume 35 which must be captured by the hood within its recess 25 while the contaminant is steadily drawn out of the hood. The recess creates a buffer zone to help insure that transient, or fluctuating, surges in the convection plume do not escape the steady exhaust flow through the vent.
It is desirable to draw off as little air from the conditioned space as possible. There are various problems that make it complicated to simply adjust the exhaust flow rate so that just enough air is withdrawn as needed to ensure all of the fumes are captured and drawn out by the hood. One problem is unpredictable cross drafts in the conditioned area. Employees might use local cooling fans or leave outside doors open. Or rapid movement of personnel during busy periods can create air movement. These drafts can shift the exhaust plume 35 sideways causing part of it to leave the suction zone of the hood allowing some of the fumes to escape into the occupied space.
Another problem is variations in the volume generation rate, the temperature and corresponding thermal convection forces, and phase change in the fumes. Generally exhaust hoods are operated at exhaust rates that correspond to the worst-case scenario. But this means they are overdesigned for most conditions. There is an on-going need for mechanisms for minimizing the exhaust rate while maintaining capture and containment of fumes.
One means for reducing the effect of cross-drafts is the use of side skirts 30 as shown in FIG. 1B. Side skirts 30, which are simple metal plates, may be affixed at the ends of an exhaust hood 46 as illustrated allowing workers to access a cooking appliance 40 from a front edge 36 of the appliance 40 without interference from the skirts 30. The skirts 30 reduce the sensitivity of the plume of fumes 35 to cross-drafts by simply blocking cross-drafts. Although only one is shown, a skirt 30 is implied on an opposite side of the hood 46 perpendicular to the line of sight of the elevation drawing.
FIGS. 1A and 1B illustrate hoods (“backshelf”) that are normally located against a wall. Another type of hood is illustrated in FIG. 2 which is called a canopy hood 60. This type of hood can have mirror image exhaust outlets as indicated at 21 (with filters also indicated at 20) or it can have an asymmetrical configuration. The canopy style hood 60 allows workers 5 to approach multiple sides of an appliance 41 such as one or more ranges. The canopy style hood is particularly susceptible to cross-drafts because of its open design.
In addition to minimizing the exhaust rate while providing capture and containment, there are many opportunities in commercial kitchens to recycle otherwise wasted energy expended on conditioning air, such as using transfer air from a dining area to ventilate a kitchen where exhaust flow rates and outdoor air ventilation rates are high. In such systems, the space conditioning or heating, ventilating and air-conditioning (HVAC) systems are responsible for the consumption of vast amounts of energy. Much of the expended energy can be saved through the use of sophisticated control systems that have been available for years. In large buildings, the cost of sophisticated control systems can be justified by the energy savings, but in smaller systems, the capital investment is harder to justify. One issue is that sophisticated controls are pricey and in smaller systems, the costs of sophisticated controls don't scale favorably leading to long payback periods for the cost of an incremental increase in quality. Thus, complex control systems are usually not economically justified in systems that do not consume a lot of energy. It happens that food preparation/dining establishments are heavy energy users, but because of the low rate of success of new restaurants, investors justify capital expenditures based on very short payback periods.
Less sophisticated control systems tend to use energy where and when it is not required. So they waste energy. But less sophisticated systems exact a further penalty in not providing adequate control, including discomfort, unhealthy air, and lost patronage and profits and other liabilities that may result. Better control systems minimize energy consumption and maintain ideal conditions by taking more information into account and using that information to better effect.
Among the high energy-consuming food preparation/dining establishments such as restaurants are other public eating establishments such as hotels, conference centers, and catering halls. Much of the energy in such establishments is wasted due to poor control and waste of otherwise recoverable energy. There are many publications discussing how to optimize the performance of HVAC systems of such food preparation/dining establishments. Proposals have included systems using traditional control techniques, such as proportional, integral, differential (PID) feedback loops for precise control of various air conditioning systems combined with proposals for saving energy by careful calculation of required exhaust rates, precise sizing of equipment, providing for transfer of air from zones where air is exhausted such as bathrooms and kitchens to help meet the ventilation requirements with less make-up air, and various specific tactics for recovering otherwise lost energy through energy recovery devices and systems.
Although there has been considerable discussion of these energy conservation methods in the literature, they have had only incremental impact on prevailing practices due to the relatively long payback for their implementation. Most installed systems are well behind the state of the art.
There are other barriers to the widespread adoption of improved control strategies in addition to the scale economies that disfavor smaller systems. For example, there is an understandable skepticism about paying for something when the benefits cannot be clearly measured. For example, how does a purchaser of a brand new building with an expensive energy system know what the energy savings are? To what benchmark does one compare the performance? The benefits are not often tangible or perhaps even certain. What about the problem of a system's complexity interfering with a building operator's sense of control? A highly automated system can give users the sense that they cannot or do not know how to make adjustments appropriately. There may also be the risk, in complex control systems, of unintended goal states being reached due to software errors. Certainly, there is a perennial need to reduce the costs and improve performance of control systems. The embodiments described below present solutions to these and other problems relating to HVAC systems, particularly in the area of commercial kitchen ventilation.